1. Field of the Invention
One embodiment of the present invention relates to fluorene compounds and to light-emitting elements, light-emitting devices, electronic devices, and lighting devices including the fluorene compounds.
2. Description of the Related Art
In recent years, research and development have been extensively conducted on light-emitting elements using electroluminescence (EL). In a basic structure of such a light-emitting element, a layer containing a substance having a light-emitting property is interposed between a pair of electrodes. By voltage application to this element, light emission can be obtained from the substance having a light-emitting property.
Since such a light-emitting element is of self-light-emitting type, it is considered that the light-emitting element is suitable as a flat panel display element because of advantages such as higher visibility of pixels than a liquid crystal display, no need to provide a backlight, and so on. Furthermore, very high speed response is also one of the features of such a light-emitting element. Besides, such a light-emitting element is highly advantageous in that it can be formed in a film form, so as to be formed thin and lightweight, and to obtain planar light emission easily. Accordingly, unlike incandescent lamps and LED lamps which are point light sources and fluorescent lamps which are linear light sources, such light-emitting elements are expected to be used for novel lighting devices including planar light sources.
Light-emitting elements using electroluminescence are broadly classified according to whether a light-emitting substance is an organic compound or an inorganic compound. When an organic compound is used as a light-emitting substance, by voltage application to a light-emitting element, from a pair of electrodes, electrons and holes are injected into a layer containing the light-emitting organic compound and thus current flows. The injection of electrons and holes into the layer containing the light-emitting organic compound forms an excited state of the light-emitting organic compound. When the carries (electrons and holes) recombine, light is emitted as a result of relaxation of the excited state to a ground state. Note that an excited state of an organic compound can be of two types: a singlet excited state and a triplet excited state, and luminescence from the singlet excited state (S*) is referred to as fluorescence, and luminescence from the triplet excited state (T*) is referred to as phosphorescence. In addition, the statistical generation ratio thereof in a light-emitting element is considered to be S*:T*=1:3.
When a light-emitting layer of a light-emitting element is formed using a phosphorescent compound, which emits phosphorescence, for suppression of the concentration quenching of the phosphorescent compound and the quenching due to triplet-triplet annihilation, the light-emitting layer is formed so that the phosphorescent compound is dispersed throughout a matrix formed of another substance in many cases. In this case, the substance used to form the matrix is called a host material, and the substance dispersed throughout the matrix like the phosphorescent compound is called a guest material.
When a phosphorescent compound is used as a guest material, a host material needs to have triplet excitation energy (an energy difference between a ground state and a triplet excited state) higher than the phosphorescent compound. In addition, the host material needs to have a carrier-transport property by which desired carrier balance can be controlled in a light-emitting layer. With the use of such a host material, characteristics of a light-emitting element can be improved.
To obtain desired physical properties of the host material, such as desired triplet excitation energy and a desired carrier-transport property, various host materials having different structures have been developed (for example, see Patent Documents 1 and 2).